The broad, long-term goal of our research program is to advance knowledge of virus-host cell interactions by characterizing cellular cofactors essential for viral infections, particularly positive- strand RNA viruses such as dengue virus (DENV) and hepatitis C virus (HCV). The current proposal focuses on DENV, which infects millions of people worldwide and poses major emerging threat to human health. Understanding how DENV interacts with the host cell is of critical importance to the development of antiviral drugs and a prophylactic vaccine, both of which are currently lacking. Building upon our previous work on in vitro hepatic differentiation and HCV infection which led to the identification of host determinants of HCV susceptibility, we propose to investigate the cellular parameters that contribute to DENV permissiveness in both monocytes and hepatic cells. We have uncovered a discrete cellular transition to DENV permissiveness during directed differentiation of pluripotent stem cells in preliminary experiments. We will scrutinize the gene profiles during this transition period to identify putative host factors required for DENV infection which in turn may be exploited as new drug targets for antiviral therapy. In addition, we will analyze the contribution of downregulated antiviral proteins to the transition to DENV susceptibility and investigate the mechanism of action for any antiviral proteins that are effective at suppressing DENV infection. The results of the proposed studies will not only provide significant insights into how DENV hijacks cellular proteins and machineries to facilitate its own replication in human cells; but also may reveal new therapeutic targets for antiviral intervention directed at important human pathogens.